The present invention relates to a method for the control of wheel slip by means of an anti-lock brake system, provided for road vehicles, with all-wheel drive and a lockable distribution differential or with a connectible drive of the second vehicle axle and a fixed coupling between front-axle and rear-axle drive. The present invention further relates to a circuit configuration for implementing this method, with sensors for the measurement of the wheel rotational behavior and with circuits for the logic combining of the sensor signals as well as for the generation of electric braking-pressure control signals. By use of these signals, valves can be changed over which are inserted into the pressure-fluid conduits of the brake system. As a result, upon the occurrence of tendencies to lock the braking pressure is controllable in the wheel brakes individually and/or jointly in wheel brakes combined to form groups.
Methods and circuitries for the electronic control of wheel slip are known. This control prevents locking of the wheels as a consequence of excessive braking force, and to thereby maintain steerability and driving stability of the vehicle even in unfavorable situations such as in the event of panic stops, on icy roads or aquaplaning, etc., and to obtain a shortest possible stopping distance. In vehicles having only one driven axle, the information required for the control, including the vehicle reference speed which corresponds approximately to the vehicle speed at optimal slip, can be derived from the rotational behavior of the individual wheels and from the sensor signals, respectively. When interpreting the sensor signals, it is advantageous to distinguish between driven and non-driven wheels, as can be taken, for instance, from German laid-open print No. 22 54 295.
In vehicles with all-wheel drive, however, the evaluation of the sensor signals and the formation of the reference variable, namely the vehicle reference speed, is considerably complicated by the coupling of all wheels by way of the drive shafts, by the transmission of drive torques and brake torques caused thereby as well as by the high moments of inertia caused by the drive elements connected to the wheels. By reason of the wheels coupled by way of the drive shafts mutually influencing each other, variations of the brake force or friction force between tires and road can no longer be recognized definitely by virtue of measurement of the rotational behavior of the individual wheels and logic combining of the measured values. In many situations, coupling of the wheels results in an almost synchronous behavior of all wheels, which renders it very difficult to detect instability, that is, an imminent locked condition.
In all-wheel driven vehicles with lockable distribution differential or in vehicles with connectible drive of the second vehicle axle and fixed coupling between the front-axle and the rear-axle drive, the described problems in interpreting the wheel rotational behavior are even more pronounced.
To avoid these difficulties, it has been known to automatically set the anti-lock control out of function as soon as the differential locks are engaged. However, this alternative bears the disadvantage that especially in situations which would necessitate both the differential locks and the anti-lock control to preserve driving stability, slip control must be dispensed with.
It has also been proposed to design a vehicle with all-wheel drive such that the brake-slip control signals immediately will cause disengagement of the locks in the differentials (German published patent application No. P 34 18 520). A principally different proposal aims at monitoring the rotational behavior of the wheels according to predefined criteria and at varying the control concept for a specific time span upon the occurrence of a wheel rotational behavior typical of overspin. As long as there is the tendency to overspin, the braking pressure control dependency on wheel slip will be discontinued, and pressure control will be determined by the wheel acceleration and deceleration alone (German published patent application No. P 3521960).
It is an object of the present invention to devise a slip control for vehicles with all-wheel drive which permits controlled braking even under unfavorable conditions, in the presence of a fixed coupling of the front-axle drive with the rear-axle drive, and which above all ensures a stable driving behavior in every situation.